Ah, I think I understand. So only run it during the "daytime" hours, as it doesn't need to vent at night.
You should run your fans 24/7 or humidity will rise, and mold may become a problem. For light proofing intake and exhaust, people use different methods, depending on their setups. Some examples are: s-bends in ducting, louvred grills, pvc tubes with bends, a series of baffles, carbon sheets etc. There are many ways to do this. Just remember that light travels in straight lines, and bounces off light-colored surfaces.
As far as I know, its the opposite: more exhaust than intake. If not, you'll be creating a positive pressure, which will push air out of any crack or opening (not the exhaust), which is bad if you want to control smell. Could be wrong, but i'm pretty sure...
If using active intakes (ie: a fan) then the intake should be slightly smaller (or weaker) than the extraction, in order to achieve negative pressure. If using passive intakes (ie: no fan, just holes), the intake area should be several times larger that the extraction, in order to avoid stressing the exhaust fan. Exactly how large will depend how powerful the exhuast fan is.